Glass fiber (and/or other fiber material such as polymeric fibers or cellulosic fibers) mats have a variety of uses, for example, as liquid or air filtration, battery separators, thermal insulation, cryogenic insulation, and electrical insulation. Glass fiber mats may be produced from glass fibers (or glass fiber strands). The production of glass fiber strands from molten glass typically involves attenuating fibers from small orifices in an apparatus that is operably linked to a glass-melting furnace. The extruded glass fibers are made into the desired diameter and length (often by mechanical means) and are collected or directly processed into mats.
Dry-laid and wet-laid processes are methods for producing glass fiber mats from glass fibers. Typically, in a dry-laid process, glass fibers are chopped and air blown onto a conveyor. In some applications, a binder is then applied to form a mat. Traditional dry-laid processes are particularly suitable for the production of highly porous (e.g., low density) mats having bundles of glass fibers. In a wet-laid process, glass fibers are dispersed in an aqueous solution, which may contain dispersants, viscosity modifiers, defoaming agents or other chemical agents. A slurry of suspended fibers or a mixture of fibers and particulates, i.e., fillers, is deposited onto a moving screen or cylinder where water is removed (usually by suction or vacuum devices). In some instances, a polymeric or inorganic binder is added for further sheet enhancement, by way of, e.g., a beater-add-processor or the binder can be applied after a sheet is semi-formed or fully formed by way of a spray, curtain coater, size press or other means know to those of ordinary skill in the art. This addition may be followed by vacuum liquid removal and/or drying by application of heat in the form of radiate, convection or conduction sources.
In an example of a dry-laid process, glass fibers mats are produced by a rotary cat process, wherein for example, small diameter streams of molten glass are caused to flow from a melting tank, and the streams are accelerated longitudinally to cause attenuation to a desired fiber diameter. The attenuated fibers are collected on a conveyor as a mat in which the individual fibers are intermeshed and randomly oriented. Such mats typically have a density, when under no load (i.e., no or substantially no external force on the mats—a load or external force of about 10 kPa or less), of from about 5.0 to about 50 kg/m3. A mat density in this range is unsuitably low for some commercial applications (such as battery separators, certain filter media, or certain insulation applications).
Certain dry-laid fiber mats have achieved higher densities by utilizing hydro-entanglement techniques (e.g., where a high-velocity jet stream of water penetrates the mat orienting a portion of the fibers in the stream's path in its direction to further entangling the mat) or other fiber mat wetting processes wherein the mat is wetted with a liquid and then compressed, the liquid in the mat suppressing the natural resiliency of the mat. However, hydro-entanglement requires the fibers penetrated by the water (and other wet methods require the mat) then be dried on an apparatus similar to what is necessary for a wet-laid fiber mat (i.e., requires a drying screen or the like). See, e.g., PCT/US97/17187, WO 98/12759. The wetting/liquid addition to the mat approach to increase density of a dry-laid fiber mat likewise requires additional equipment for adding the liquid and requires the liquid remain present in the compressed fiber mat to continue suppression of the fiber mat's natural resiliency (i.e., liquid must remain to keep the fiber mat density relatively high). See, e.g., U.S. Pat. No. 5,468,572. Some dry-laid mats have been subjected to needle-punching in attempt to increase densities. However, needle-punching alone is not sufficient to achieve the densities desired in a dry-laid fiber mat and such treatment adds holes to the mat and requires further equipment/steps for processing the mat product.
Although wet-laid fiber mats may meet such density requirements, the wet-laid process is more expensive and requires additional steps that often destroy the fiber's properties. In addition, dry-laid fiber mats are not subjected to degradation from pulping, pumping, cleaning, chemical additives, and/or pressure damage to which wet-laid fiber mats are subjected. Thus, it is desirable to increase the density of bonded or unbonded, dry-laid glass fiber mats to make such mats suitable for an expanded number of uses. Likewise, it would be useful to have dry-laid fiber mats with densities previously only achieved using wet-laid processes, or by including binders with dry-laid fiber mats, by adding liquids to dry-laid fiber mats, or by treating dry-laid fiber mats with hydro-entanglement and/or needle-punching steps. Accordingly, methods of enhancing the physical properties (such as, density, thickness, compression resiliency and/or tensile strength) of dry-laid fiber mats (and wet-laid fiber mats in certain circumstances) are needed. In addition, dry-laid fiber mats wherein relatively high density values are achieved without the need for needle-punching, hydro-entanglement, and/or wetting with a liquid to achieve such densities are desirable.